gui: add persistent notification and run NfdService in foreground to improve stability

Change-Id: I47e7ff07888e4638a75ddb607d335511467a7779
refs: #4766
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index 47032af..eb7b812 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -114,4 +114,8 @@
     <string name="fragment_route_details_next_hops">List of next hops</string>
     <string name="fragment_route_route_name_title">Route Name</string>
     <string name="ping_client_prefix_hint">Prefix (e.g., /ndn/edu/arizona)</string>
+
+    <string name="notification_content_title">Status</string>
+    <string name="notification_content_text">NFD is running</string>
+    <string name="channel_description">NDN Android notification.</string>
 </resources>